Messaging can be defined as time-delayed communication or communication in which the origination of the communication occurs asynchronously from its reception. For example, letters and public notices are traditional form of messaging. Messaging also includes traditional forms of publishing, whether text/image-based (e.g. memoranda, books and periodicals), audio-based (e.g. tapes and compact discs) or audio/visually-based (e.g. movies). Other forms of messaging include voice mail, including multi-user systems and answering machines, and electronic counterparts of traditional messaging, for example telegrams, cablegrams, faxes, telex, electronic mail, asynchronous computer-based conferencing (i.e. bulletin-boards) and electronic publishing.
Because messaging is communication across time, messaging requires storage of the message content. The way in which the message is stored can be used to distinguish types of messaging.
"Store-and-forward" messaging is one type of electronic messaging system. In store-and-forward messaging, a message is transmitted from its sender to its recipient through at least one "message transfer agent" (MTA). The message can be relayed through as many successive MTAs as are needed for the message to reach its final destination. Each MTA independently chooses the next MTA to which to route the message, based on information carried along with the message, such as its intended recipient, together with information held by the MTA, such as routing tables. If a message is directed to more than one recipient, an MTA can multicast the message by sending copies of the message to separate MTAs for forwarding to each intended recipient. When the message reaches an MTA that has direct access to the recipient's mailbox, the message is deposited in local area network (LAN) storage associated with the recipient's mailbox. The LAN storage associated with recipient's mailbox is comprised of message stores for storing messages deposited therein.
During the messaging transmission process, each MTA temporarily stores the message, until the message is transferred to another MTA or deposited in the LAN storage associated with the recipient's mailbox. Thus, the MTAs constitute a succession of storage locations and the message is forwarded from MTA to MTA until ultimately reaching the recipient's mailbox.
Referring to FIG. 1, the general structure of store and forward messaging as applied to LAN messaging and wide area network (WAN) messaging is shown. With respect to LAN messaging, a message can be sent from a sender at a sender station 1 to a recipient at a recipient station 2 through a LAN 3. The message is transmitted from the sender station 2 through an MTA 6 in LAN 3. MTA 6 has a message store 12 that stores the message temporarily until the MTA, acting on information carried along with the message, routes the message. Because MTA 6 has direct access to the recipient's mailbox 4, MTA 6 deposites the message into LAN 3 storage associated with the recipient's mailbox 4. The recipient can then access the message in his mailbox 4 from the recipient station 2.
With respect to WAN messaging, a series of LAN systems may be connected together (not shown) or LAN systems may be connected through an intermediate WAN (shown in FIG. 1) to permit WAN messaging. As shown in FIG. 1, a message can be sent from the sender at sender station 1 to a recipient at a recipient station 11 through MTA 6 in LAN 3, MTA's 7, 8 and 9 in a WAN 17 and an MTA 10 in a LAN 5. MTA's 6, 7, 8, 9 and 10 have messages stores 12, 13, 14, 15 and 16, respectively, that store the message temporarily until the MTA, acting on information carried along with the message, routes the message. The message is transmitted from sender station 1 to MTA 6 in LAN 3. MTA 6 routes the message to MTA 7 in WAN 17. MTA 7 in turn routes the message to MTA 8 in WAN 17 and MTA 8 in turn routes the message to MTA 9 in WAN 17. MTA 9 routes the message to MTA 10 in LAN 5. MTA 10 has direct access to the recipient's mailbox 18 and deposits the message into the LAN 5 storage associated with the recipient's mailbox 18. The recipient can access the message in his mailbox 18 from the recipient station 11.
Because store-and-forward messaging is an application specific overlay on a network, it has limitations. For example, the costs involved are spread only among the users of the store-and-forward messaging application. Moreover, the scope of messaging is limited to users of that application. Finally, diverse message formats and conversion from one format to another them are not readily accommodated.